In the manufacture of molded records, such as conventional audio records or the more recently developed capacitive electronic discs, a thermoplastic material is molded between a pair of metal disc-shaped parts referred to as stampers. The stampers have defined in their molding surface a spiral information track which contains a surface relief pattern corresponding to the program information desired to be reproduced on playback of a molded record pressed with the stampers.
Stampers are the end product of a multi-step process which is broadly referred to as matrixing. The first step of the matrixing process is to record the program information on a magnetic tape. The magnetic tape is used to control a tool which cuts a recording substrate with a spiral information track having a surface relief pattern corresponding to the recorded program information. The recording substrates which are typically employed are flat metal discs which have a layer of a relatively soft material formed on one surface thereof into which the spiral information track is cut.
The recording substrate having the spiral information track is replicated in a series of electroforming steps. In the first of the electroforming steps, a metal, such as nickel, is electrodeposited on the recorded surface of the recording substrate until a self-sustaining matrix of a predetermined thickness is obtained. The resulting electroformed matrix, called a master, is then separated from the recording substrate and replicated. After a predetermined thickness of metal has been deposited on the master, the resulting matrix, referred to as a mother, is separated from the surface of the master. The mother then in turn likewise replicated to produce a part referred to as a stamper. The stamper is a negative replica of the recording substrate. The stamper will have formed in its molding surface a mirror image of the spiral information track which was originally cut into the recording substrate and will be of a flat disc-shaped configuration like the recording substrate.
The stampers obtained directly from the matrixing process described above are not suitable as formed for use in the presses conventionally employed in the molding of records. The flat stampers obtained from the above-described matrixing process are in effect stamper blanks which must be subjected to a number of edge shaping steps in order to have the required configuration for mounting on the molding platens of conventional record molding presses. The flat stampers from the matrixing process are trimmed. A center hole is cut into the flat stampers which has a precise diameter and is concentric with the information area of the stamper. The outer edge of the flat stamper is also trimmed to a precise diameter concentric with the information area of the stampers. After the stampers are trimmed, the inner and outer edges are further shaped in stamping operations. The inner edge is stamped to form a tube-like extension about the center hole which is used to secure the center of the stamper to the molding platen. The outer edge of the stamper is likewise shaped in a stamping operation which results in the outer edge being shaped so as to mold an outer bead on the molded record as well as to form the flash and provide a land area for molding. In addition, a circumferential section is formed about the outermost edge for engaging the outer edge of the molding platen.
The requirement for shaping of the inner and outer edges of the stampers is a source of considerable problems. It is highly desirable that the stampers be as hard as possible so as to resist scratching, dents and other types of mechanical damage during handling and pressing. However, since the flat stampers from the matrixing process are shaped in stamping operations as noted above, it is also necessary that the metal of the stampers be sufficiently ductile to permit deformation during stamping without cracking occuring, either as the stampers are shaped, or thereafter when the stampers are in the mold. The requirement for ductility of flat stampers manufactured by the above-described matrixing process has resulted in there being an undesirable tradeoff of hardness for ductility. Furthermore, it has been found that despite compromises being made in the hardness in order to obtain ductility, almost all of the problems encountered with conventional stampers during the pressing of records can be directly traced back to the stamping steps used to shape the inner and outer edges. The stampers having the edges shaped by stamping as described above, when used in the molding of records, usually fail as a result of fatigue cracking at the outer edge of the stampers. This is believed to be due to the stresses that are introduced into the shaped outer edge during stamping and to thinning of the metal of the shaped outer edge. Occasionally cracking problems are also encountered at the shaped inner edge.
What would be highly desirable would be an improved method for the manufacture of stampers having a relatively high hardness in the recorded areas and increased resistance to fatigue cracking at the formed inner and outer edges.